The primary aim of this U01 project is the technical development, deployment, and evaluation of hardware and software technology that will enable population-scale, longitudinal measurement of physical activity using common mobile phones. Mobile phones available in Asia and soon in the U.S. already include internal accelerometers and low-power wireless communication capabilities. This study will investigate how to use these computing devices for accurate measurement of physical activity type, intensity and bout duration. By exploiting consumer expenditures on phones that many Americans will purchase, maintain, and carry, it may be possible to run large-scale studies where the physical activity of hundreds of thousands of participants is measured and remotely monitored for months or years at an affordable cost. Wireless accelerometers designed at MIT will be redesigned so that they can send data to common mobile phones available in 2011. Laboratory testing using the current version of the sensors will be used to compare the relative information gain that can be obtained by combining the phone accelerometer data and data obtained by wearing one or more wireless sensors on different convenient body locations (e.g., in a watch or bag, on a shoe, at the hip, etc.). Optimal but practical configurations of accelerometers will be determined so that software running on the mobile phone can automatically detect specific physical activities such as brisk walking, running, cycling, climbing stairs, sweeping, playing sports, etc. Technical challenges that will be addressed by the sensor and software design include, (1) obtaining practical battery life, (2) acquiring physical activity data at high temporal resolution, (3) enabling person-specific customization of the detection algorithms, (4) addressing practical end-user concerns about ergonomics, comfort, and social acceptability, (5) permitting real-time and low-cost remote monitoring and maintenance for studies with hundreds of thousands of phone users, and (4) enabling use of other off-the-shelf sensor devices, such as heart rate monitors, as they become available. A participatory design process will be employed to develop strategies for obtaining longitudinal compliance from typical phone users. After two rounds of iterative technical development, each with laboratory validation conducted at Stanford, the technology will be deployed with 50 typical phone users for 10 months. Validity relative to self report, acceptability, and longitudinal compliance will be measured.